A pitching angle is used by an autolevelizer system that adjusts the optical axis of headlights of a vehicle or the like. Conventionally, there is a method of calculating the pitching angle by obtaining the heights of the front wheels and rear wheels by measuring the angle of the suspension arms (see Patent Document 1, for example), or a method of calculating the pitching angle by receiving radio waves reflected off the road surface at a plurality of places of a vehicle and by using phase differences between the radio waves received (see Patent Document 2, for example).
The road surface gradient is used by a car navigation system and the like for correcting an error produced in the vehicle turning angle a gyro detects during driving on a hill. Conventionally, the road surface gradient is estimated from the acceleration occurring in the vehicle, which is detected with a single axis acceleration sensor, the acceleration in the direction of travel of the vehicle obtained from the wheel speed, and prescribed vehicle specifications (the vehicle weight, a spring constant of the suspension and the like) (see Patent Document 3, for example).